During use, the properties of a rope decrease. In some applications, e.g., when the rope is used as mooring line, the rope suffers from tension-tension fatigue. That is, the rope is subjected to a cyclic increase and decrease of tension, and this has been found to detrimentally affect the properties of the rope. In other applications, e.g., where the rope is used over pulleys, the rope suffers from bending fatigue. That is, the properties of the rope deteriorate when the rope is subjected repeatedly to bending.
One of the key concerns of a rope user is to determine when a rope should be replaced. Replacing a rope entails substantial costs and effort, however. Not only the costs of the new rope need to be factored in, but also the costs associated with down-time of the unit wherein the rope is used, and the labour costs associated with the replacement. Therefore, it is undesirable to replace a rope too soon, that is, substantially before the end of its lifetime. On the other hand, the situation that a rope breaks or otherwise fails is unacceptable, and needs to be prevented.
Therefore, within the rope field methods have been developed to test the properties of the rope while it is in use, to allow the rope user to determine when the rope should be replaced. Testing the properties of a rope while it is in use is indicated in the field as non-destructive testing.
A first non-destructive testing method known in the art is magnetic field testing, wherein the object to be tested is brought into a magnetic field, and the presence of defects is detected through areas of flux leakage in the rope. A further method is eddy current testing wherein an alternating electrical current is passed through a coil producing a magnetic field. When the coil is placed near a conductive material, the changing magnetic field induces current flow in the material. These currents travel in closed loops and are known as eddy currents. Eddy currents produce their own magnetic field that can be measured and used to determine the presence of flaws in the rope.
While the methods specified above have shown their value in wire ropes, they are not directly applicable to synthetic ropes, as they rely on the magnetic and electrically conductive properties of the rope. Synthetic ropes are in principle very attractive to replace wire ropes in numerous applications, as they have a number of advantages, including lighter weight for the same strength, insensitivity to corrosion, and lower maintenance requirements. However, for synthetic ropes to be used in high risk applications, the availability of a method for testing the rope properties in use is required.
Methods for non-destructive testing of synthetic ropes are known. U.S. Pat. No. 6,886,666 describes the non-destructive testing of metallic and synthetic load bearing members. The load bearing member comprises a first, structural material (which can be a synthetic fiber) and at least one element of a second material which has a distinguishing characteristic from the first material.
The second material is used for detecting strain on the load bearing member.
The disadvantage of using a tracking fiber which is made from a different material is that the tracking fiber which is used to assess the condition of the load bearing member inherently possesses characteristics different from the first, structural material.
To overcome this disadvantage it would be advantageous to use a second material within the rope for assessing the condition of the rope which has more or less the same characteristics and properties (e.g. with regard to mechanical properties and behavior in the rope) as the first material.
The present invention provides such a method.